1. Field of the Invention
The present invention is directed to a facility for generating a uniform magnetic field, particularly for generating a basic magnetic field in a magnetic resonance examination apparatus, particularly a medical magnetic resonance tomography apparatus, of the type having a shim device for improving the homogeneity of the magnetic field. The invention also is directed to a magnetic resonance examination apparatus and to a set of ferromagnetic articles for use in shimming. Further, the invention is directed to a method for improving the homogeneity of a magnetic field, particularly the magnetic basic field in a magnetic resonance examination apparatus, particularly a medical magnetic resonance tomography apparatus, of the type wherein multiple ferromagnetic articles are introduced into the magnetic field for influencing the field.
2. Description of the Prior Art
The basic principles relating to shimming a magnetic field are described in conjunction with nuclear magnetic resonance tomography in, for example, the book by Heinz Morneburg, xe2x80x9cBildgebende Systeme fxc3xcr die medizinische Diagnostik,xe2x80x9d 3rd edition, 1995, page 520. xe2x80x9cShimmingxe2x80x9d means the elimination of basic field inhomogeneities caused by manufacturing tolerances and ferromagnetic articles distributed in the environment of the tomography apparatus.
To this end, for example, the field is first measured using at least one probe in an adequate number of pickup locations uniformly distributed on the surface of the spherical homogeneity volume. The field values are entered into a computing program that calculates a suitable arrangement of iron plates to be attached in the magnet bore or cylindrical tube. A check measurement also is implemented after assembly. This procedure usually must be repeated once or twice until a satisfactory shim result has been achieved.
It is also possible to homogenize the field with correction coils or shim coils (active shimming). Methods and devices of this type are disclosed, for example, in U.S. Pat. Nos. 4,680,551 and 6,002,255.
The introduction of iron plates or differently shaped pieces of iron is referred to as passive shimming. As disclosed in U.S. Pat. No. 5,400,786, an annular shim mechanism can be attached in the inside of the cylinder pipe given a magnetic resonance examination apparatus with a cylindrical device for generating the magnetic basic field (closed system).
U.S. Pat. No. 5,431,165 discloses the attachment of shim devices to the upper pole shoe and to the lower pole shoe for an open magnet system.
European Application 0 677 751 discloses the use of correction rings for shimming formed by windings of a number of turns of iron or steel that are electrically insulated from one another. Eddy currents generated by the gradient coils of the magnetic resonance tomography apparatus are to be minimized as a result.
U.S. Pat. No. 6,218,839 B1 and PCT Application WO 88/08126 disclose inserts for introduction into the opening of a magnetic resonance tomography apparatus that are employed for shimming. The inserts have pockets at predefined locations into which an iron piece can be placed or the pocket can be left empty, as needed.
An object of the present invention is to provide a shimming facility a method with which a magnetic field can be even more precisely homogenized than with known devices and methods.
This object is achieved in a facility for generating a uniform magnetic field, having a shim mechanism with a guide device attachable in the region of the magnetic field which carries a number of ferromagnetic articles in a movable manner. The ferromagnetic articles can be movably attached to or in the guide element. The guide device, along which the ferromagnetic articles are movable, makes it possible to select the positions at which a ferromagnetic article should or can be introduced more freely than in known devices. In particular, it is not necessary to provide pocket-forming spacers or the like between the ferromagnetic articles. After the ferromagnetic articles have been moved to the desired position along the guide device, they can be fixed in this position. Suitable fixing elements can be present for this purpose.
In particular, the shim mechanism is present in the facility separately from the magnet system for originally generating the magnetic field, for example a coil or a permanent magnet.
The fixing of the ferromagnetic articles at their desired location in or at the guide device can occur by means of so many ferromagnetic articles and, possibly, non-ferromagnetic articles as well, being attached at or in the guide device so that the articles touch one another, so that a specific article is held by at least two neighboring articles that can be ferromagnetic or non-ferromagnetic. In this case, it suffices to fix only the articles present at the end of the guide device with a separate fixing arrangement. The guide device can be linear.
The ferromagnetic and/or non-ferromagnetic articles can be fashioned as gliding or rolling bodies that can be displaced or rolled along the guide device.
In a preferred embodiment, the ferromagnetic articles, and the non-ferromagnetic articles which may be optionally present as spacers between two ferromagnetic articles are implemented as balls, and the guide device is a pipe or tube for accepting the balls. For example, the outside diameter of the balls is matched to the inside diameter of the tube.
In another preferred embodiment, at least two articles having ferromagnetic properties differing from one another, particularly having saturation magnetizations that differ in magnitude from one another, are attached to or introduced into the guide device.
This especially preferred embodiment has the advantage that an especially good homogenization of the magnetic field is possible. In the known devices for homogenizing the magnetic field, there is only the degree of freedom in the optimization of introducing or not introducing a ferromagnetic article, for example a shim plate, at a specific, pre-defined position. This quasi-digital situation (0 or 100%) leads to unavoidable rounding errors during the optimization. The idea of resolving the quasi-digital situation with magnetic properties of differing intensities is particularly advantageous in conjunction with the employment of balls guided in a tube as ferromagnetic articles, since the use of balls of different sizes would thereby not be practical. The idea of employing articles having different ferromagnetic properties for the shimming leadsxe2x80x94in addition to the guide devicexe2x80x94to a further degree of freedom in the optimization during the shimming. The position of an influencing article as well as the strength of its influence on the magnetic field thus can be more freely selected than with known devices.
The idea of setting the strength of the influence of an article employed for shimming on the basis of its ferromagnetic properties also can be employed independently of whether the articles are movable along a guide device. Instead of being movable in or at a guide device, such articles could also be attached immobile in pre-fixed positions of a carrying structure of some type or other.
Preferably, the shim mechanism has a set of a number of ferromagnetic articles allocated to it from which ferromagnetic articles can be selected as needed for attachment to or, respectively, introduction into the guide device. The set contains a number of types of article, with each type having a magnitude of a ferromagnetic property different from the other type or types, particularly a different magnitude of the saturation magnetization.
In particular, the magnitude of the ferromagnetic property changes in equidistant steps from one type to the next. For example, the types represent a saturation magnetization of 0% (non-ferromagnetic article), 20%, 40%, 60%, 80% and 100% of a specific absolute value.
For realizing different ferromagnetic properties, the articles have material compositions that differ from one another, for example, different alloys, particularly different alloys of iron, and/or nickel and/or cobalt.
Different grades of steel also can be utilized.
The aforementioned object also is achieved in accordance with the invention in a magnetic resonance examination apparatus having a facility for generating a uniform magnetic field as described above. In this case, the facility serves the purpose of generating the basic magnetic field of the magnetic resonance examination apparatus.
The above object also is achieved according to the invention by a set of a number of ferromagnetic articles for employment with a shim device, particularly with a shim device of a magnetic resonance examination apparatus, the set containing a number of types of identically shaped articles, each type exhibiting a magnitude of a ferromagnetic property different from the other type or types, particularly a different magnitude of the saturation magnetization. Because the articles have the same shape, they are easily suitable for each other.
To this end, the articles are also preferably of the same size.
Also preferably, the magnitude of the ferromagnetic property changes in equidistant steps from one type to the next.
In particular, the articles have material compositions that are different from one another.
The above object also is inventively achieved in a method including the steps of providing at least two articles having ferromagnetic properties that differ from one another, particularly having saturation magnetizations differing in magnitude from one another, and introducing the articles into the magnetic field.
Articles having the same shape and/or the same size are thereby preferably employed in the method.
In particular, the articles can be provided with material compositions that differ from one another.
The preferred embodiments and advantages presented in conjunction with articles having ferromagnetic properties differing from one another in conjunction with the facility of the invention for generating a magnetic field apply analogously to the method of the invention.
In an embodiment of the method wherein a measured field distribution is identified, a field distribution that is optimized in homogeneity is calculated, with the magnetic properties of the available articles and, optionally, the number and/or the positions of the articles in the magnetic field being employed as optimization parameters, and wherein respective articles having the calculated magnetic property are introduced into the magnetic field at prescribed positions or at positions calculated in this way.
Preferably in the optimization proceeds on the basis of more than two different types of article being available, with each type having a magnitude of a ferromagnetic property that is different from the other type or types, particularly a different magnitude of the saturation magnetization.
Particularly the optimization proceeds on the basis that the magnitude of the ferromagnetic properties changes in equidistant steps from one type to another.